A. Technical Field
The present invention relates to a process and an apparatus for production of a water-absorbent resin.
B. Background Art
Water-absorbent resins are widely utilized for various uses such as sanitary articles (e.g. disposable diapers, sanitary napkins, adults' incontinent products) and water-retaining agents and are produced and consumed in large quantities.
As prior processes for production of these water-absorbent resins, there are known arts such as aqueous solution polymerization. For example, there can be cited such as: a process comprising the step of polymerizing an aqueous solution containing a hydrophilic monomer while fracturing the resulting polymer gel by stirring (refer to JP-A-034101/1982); and a process comprising the step of statically polymerizing an aqueous solution containing a monomer (refer to JP-A-156102/1987, JP-A-126310/1989, JP-A-174414/1991, JP-A-175319/1992, JP-A-236203/1992). Particularly, in the case where the static polymerization is continuously carried out for the production, for example, there can be adopted a process comprising the step of continuously supplying the aqueous monomer solution onto an endless belt to cause the aqueous monomer solution to run a polymerization reaction.
In such a process for production of a water-absorbent resin, it is actually impossible to mix the aqueous monomer solution, as supplied onto the belt, and a polymerization initiator together, and it is therefore necessary to mix the aqueous monomer solution with the polymerization initiator before supplying the aqueous monomer solution onto the belt. However, if, when the aqueous monomer solution and the polymerization initiator are beforehand mixed together, the mixing or supplying takes time, or a portion of the resulting mixed liquid stagnates in the piping, then there may occur problems such that a polymer adheres to the inside of the piping or apparatus (through which the mixed liquid is passed) and grows there before the mixed liquid is supplied onto the belt, so that the piping is clogged up. Accordingly, it is necessary to rapidly carry out the mixing of the aqueous monomer solution and the polymerization initiator and the supply of their mixture onto the belt, and further it is necessary that the mixing is carried out sufficiently and uniformly. Particularly in the case where a high-concentration or high-temperature aqueous monomer solution and the polymerization initiator is mixed together, extreme difficulties are involved, because the induction period of the polymerization is short and also because the polymerization is fast.
For example, in cases of such as inline mixing and spray mixing which are used as common arts to mix liquids together, a comparatively good mixed state may be obtained, but a dead space is produced in a mixer so easily that a portion of the resulting mixed liquid stagnates in this dead space. Therefore, a polymer forms and grows to thus clog up the mixer.
Thus, there have been proposed some arts which specify the method for the mixing of the aqueous monomer solution and the polymerization initiator in the continuous polymerization, thereby aiming at inhibiting the clogging of the piping caused by the polymerization and at improving the workability.
For example, there have been proposed such as: (a) an art in which the polymerization initiator is supplied at a flow rate of a specific ratio to a flow rate of the aqueous monomer solution into a supply pipe through which the aqueous monomer solution is flowing (refer to Japanese Patent No. 2679280); (b) a method in which the aqueous monomer solution and the polymerization initiator are mixed together by the force of a jet of a fluid using a mixing head having at least two jet nozzles without any driving part (refer to JP-A-032514/1981); and (c) an art in which the aqueous monomer solution and the polymerization initiator are supplied through their respective different supply pipes and then caused to flow into each other to thereby be mixed together in a position of just before being discharged into a polymerization machine (refer to JP-A-240903/1999).
However, none of the aforementioned prior arts can be said to be sufficient, but all the aforementioned prior arts still have problems.
For example, in the case where the flow rates of the aqueous monomer solution and the polymerization initiator are merely adjusted like in the art (a) of Japanese Patent No. 2679280, a uniformly mixed state cannot be obtained immediately after the supply of the polymerization initiator (in more detail, because the flow rates are adjusted in the range near an equal rate). Therefore, the properties of the final product are inferior. It takes such a long time to make the mixed state uniform by this method as to need a long distance, namely, long piping, from the mixing to the supply to the polymerization machine. As a result, a polymer forms and then adheres to the inner surface of the piping and grows there to thus clog up the inside of the piping.
As to the mixing head used in the art (b) of JP-A-032514/1981, there is a possibility that: the monomer may polymerize near the jet nozzle to thus clog it up; or a portion of the mixed liquid may stagnate in the inner space of the mixing head, so that a polymer may adhere and grow to thus hinder the mixing or clog up the jet nozzle.
As to arts involving the confluence in the position of just before the supply into the polymerization machine such as the art (c) of JP-A-240903/1999, the mixing makes progress almost on the polymerization machine. Therefore, almost no polymer forms in the piping. However, there is a long way from the achievement of the uniformly mixed state.